1. Yes, you'll have to reach out to Hifiman customer service directly (
@TeamHiFiMAN) and they'll provide instructions on how to enter your order in. They'll ask you which HEK headstrap you'd like to order (v1, v2, or se). Before doing that, I would test out to see whether the screws holding the headstrap can be unfastened first. Be careful, I believe that they strip pretty easily. Better get a jeweler screwdriver set (only a few bucks) at a local hardware store if you don't have any other precision screwdrivers available. I replaced the headstrap on my HEKv1, so let me know if you need any help.
Quick tip: For the end portion of the headstrap assembly that has a beaded protrusion used to catch the indentations of the headband, be sure to bend that end portion outward slightly using a plier. Be sure to test it on the headband before finishing screwing the headstrap assembly together with the headstrap sliders. The reason why is that the stock configuration of the beaded end portion does not catch the headband holes (i.e. the headband sliders move up and down without resistance). I learned this the hard way and had to go back and bend the end portions. If you bend the end portions outward too much, then you won't be able to move the headstrap slider since it will be caught at that one position. You'll have to test it out to see how much you'll like the headband sliders to move.
2. Full leather earpads are sold by a third-party Chinese seller on ebay and retaining rings are not included if I remember correctly. You'll have to take the retaining rings from the existing ear pads to use in the leather earpads. However, there have been fit issues with the leather earpads where the inner lip is supposedly too thick, so the retaining ring cannot clasp onto the earcup. You can message the ebay seller to see whether that issue has been resolved.
3. That's up to you. I have a HEKv1 and I enjoy it more out of the Liquid Platinum than my THX AAA 789. I haven't heard the Asgard or A90 amps. IMO, the best bass impact came from running it from speaker taps on my Marantz MM7025. Unfortunately, the noise floor is audible (just barely) on the Marantz power amp. On my Bryston 3B-ST (speaker power amp for my HE-6), I can definitely hear the noise floor with the HEKv1. With a sensitivity of 90 mW/dB, choices for speaker amps will be limited for the HEK. Then again, HEK's are not really known for their slam.